1. Field of the Invention
The present invention is generally relates to a semiconductor device and a manufacturing method therefor. More particularly, the present invention relates to a semiconductor device having PN junction and a manufacturing method therefor. The present invention is applied to, for example, a variable capacitance element for use in voltage controlling the frequency of a communication VCO oscillator or filter, to an Esaki diode for use in an switching element in a microwave oscillator, microwave amplifying circuit or logic circuit, or to a light emitting diode (LED) or semiconductor laser diode for use in a commercial, vehicle equipped or industrial display.
2. Related Arts
Conventionally, a varicap diode which controls the capacity of a depletion layer caused to PN junction by controlling applied voltage has widely been used for a VCO (voltage controlled oscillator) unit for a communication instrument or the like. The capacity of the depletion layer of a variable capacitance element is used as the capacity of a parallel-plate capacitor which uses the ends of depletion layers spread in a P-type layer and an N-type layer respectively formed in course of junction as a distance thereof and also uses the cross sections of a P-type layer and an N-type layer positioned at the ends of the depletion layers respectively as a plate area thereof. This capacity of the depletion layer is in proportion to the cross section of the parallel plates (i.e., the plate area) and in inverse proportion to the distance. The characteristics of the above VCO unit depend on what type of varicap diode is selected. For example, as required abilities, the VCO unit must be able to reduce and stabilize distortion in the modulation or demodulation of communication signals, greatly change the oscillation frequency with a small input voltage change, and save power consumption by means of low voltage driving. In contrast with these requirements, the varicap diode must be able to have a high linearity of C-V (the relationship between capacity of the depletion layer and applied voltage) curve, have a wide capacity variation width, and obtain sufficient capacity variation even with a low voltage. Generally, the spread of a depletion layer is controlled by the applied voltage, and the spread of the depletion layer is in proportion to V.sup.1/3 of the applied voltage in graded junction type and in proportion to V.sup.n (n=2 to 3) of the applied voltage in hyper abrupt junction type. Accordingly, in the varicap diode, PN junction of hyper abrupt junction type in which the spread of the depletion layer varies more greatly against the same applied voltage compared with the graded junction type is preferable.
Conventionally, as a manufacturing method for the above varicap diode, epitaxial planar type has been used. In the epitaxial planar type, a high impurity concentration semiconductor substrate on which a low impurity concentration epitaxial layer of the same conductivity type is grown is prepared, and a deep impurity diffusion layer of the same conductivity type as that of the epitaxial layer is formed from the surface of the epitaxial layer, and furthermore, a shallow impurity diffusion layer of different conductivity type from that of the epitaxial layer is formed to obtain PN junction.
On the other hand, a light emitting diode and a semiconductor laser diode are requested to dispose a P-type degenerate semiconductor and an N-type degenerate semiconductor, both of which have a high impurity concentration, close to each other, to thereby raise recombination efficiency in the transition region.
In the Esaki diode used for microwave communication, as ON/OFF current ratio and switching speed in switching must be raised, the above two degenerate regions are disposed as close as possible to each other to form PN junction.
However, in the conventional varicap diode, as two different conductive impurities are diffused from the substrate surface by means of ion implantation and annealing one by one in the epitaxial planar type, the impurity diffused earlier further diffuses under the effect of the heat received during the impurity diffusion process of the other impurity. As a result, the impurity distribution called "step type" is formed as illustrated in FIG. 9. Therefore, the epitaxial planar type has a problem that the hyper abrupt type junction having a desired depletion layer width or transition region width is hard to obtain.
In the Esaki diode, as diffusion is achieved by alloying indium with Ge and the degenerate regions are positioned as close as possible to each other to form PN junction, there is a problem that the hyper abrupt type junction having a desired depletion layer width or transition region width is hard to obtain.
Furthermore, in the light emitting diode and a semiconductor laser diode, it is requested to position the P-type degenerate semiconductor and N-type degenerate semiconductor both of which have a high impurity concentration close to each other. In the epitaxial planar type, however, there is a problem that the hyper abrupt type junction having a desired depletion layer width or transition region width is hard to obtain.